Many consumer electronic devices are now being built with touch sensitive surfaces (track pad or touch screen), for use with finger or stylus touch user inputs. These devices range from small screen devices such as mobile phones and car entertainment systems, to mid-size screen devices such as notebook computers, to large screen devices such as check-in stations at airports.
In computing, multi-touch refers to a touch sensing surface's ability to recognize the presence of two or more points of contact with the surface. This plural-point awareness is often used to implement advanced functionality such as pinch to zoom or activating predefined programs (Wikipedia, “multi-touch”). The Windows 8 operating system from Microsoft Corporation requires a touch screen supporting a minimum of 5-point digitizers. WINDOWS® is a registered trademark of Microsoft Corporation.
The present invention relates to light-based touch sensitive surfaces. Light-based touch sensitive surfaces surround the surface borders with light emitters and light detectors to create a light beam grid above the surface. An object touching the surface blocks a corresponding portion of the beams.
Reference is made to FIG. 1, which is a diagram of a prior art, light-based touch screen having 16 LEDs and 16 PDs. Screen 801 in FIG. 1 is surrounded by emitters 101 along two edges and photodiode (PD) receivers 201 along the remaining two edges, which together enable a lattice of light beams 300 covering the screen.
Light-based touch detection systems are unable to accurately recognize many instances of two or more points of contact with the surface. Reference is made to FIGS. 2 and 3, which are illustrations of instances of ambiguous multi-touch detections in prior art touch screens. FIGS. 2 and 3 show different instances of two, diagonally opposed touches 901 and 902 that are ambiguous vis-à-vis the light grid of FIG. 1. As shown in FIGS. 2 and 3, the same light beams are blocked in both instances.
There is further ambiguity when more than two objects touch the screen simultaneously. Reference is made to FIGS. 4 and 5, which are illustrations of instances of ghosted touches in prior art touch screens. The two-touch cases shown in FIGS. 2 and 3 are also ambiguous vis-à-vis the three-touch case, 901-903, shown in FIG. 4, and vis-à-vis the four-touch case, 901-904, shown in FIG. 5. In each of the cases illustrated in FIGS. 2-5, row and column PDs a-h show an absence of light in the same locations. The ambiguity illustrated in FIGS. 4 and 5 is caused by “ghosting”, which refers to an effect where the shadow of a first object obscures a second object and prevents the second object from being detected.
Light-based touch screens have many advantages over other touch sensor technologies such as capacitive and resistive solutions. Inter alia, light-based touch screens enable lower bill-of-materials cost than capacitive solutions, especially for large screens. Light-based touch screens are also superior to capacitive and resistive solutions in that a light-based touch screen does not require an additional physical layer on top of the screen that impairs the screen image. This is an important advantage for devices employing reflective screens where the brightness of the screen image depends on reflected light, rather than a backlight. Reference is made to U.S. Pat. No. 8,674,966 for ASIC CONTROLLER FOR A LIGHT-BASED TOUCH SCREEN, incorporated herein by reference in its entirety, which teaches faster scan rates for light-based touch screens than are available using prior art capacitive screens. It would be advantageous to provide a light-based touch screen that is operative to detect multi-touch gestures and that is compatible with the requirements of the Windows 8 operating system.
One drawback of prior art light-based touch screens is the need to accommodate the numerous light emitters and light detectors along all four edges of the screen. This requirement makes it difficult to insert light-based touch detection into an existing electronic device without significantly changing the layout of the device's internal components. It would be advantageous to reduce the number of components required and to enable placing them in a limited area rather than surrounding the entire screen. Reducing the total number of light emitters and light detectors required has the added benefit of reducing the bill-of-materials (BOM).